CPU Power

Your Mac has one or more central processing units (CPUs)—the computer’s “brains.” The CPU is where all the calculations happen that enable your Mac to do everything from displaying a single dot on the screen to surfing the web and editing video. Since calculations are what a computer is all about, it stands to reason that the more CPU power you have, the faster your computer will be. However, in this case “power” is a slippery notion, and direct comparisons between Macs with different sorts of CPUs get tricky. Several factors come into play.

RAM

The second major factor that determines your Mac’s speed is random-access memory, or RAM. RAM is fast memory in which program code and data are stored and retrieved during use. Because it’s constantly being accessed, RAM is, in some respects, more important than raw CPU power. After all, many of the things you do with your Mac—word processing, reading email, listening to music, and so on—have modest CPU needs, and so could run at virtually the same speed on an old, slower Mac as on a new, faster one. But even the newest and shiniest Mac can feel painfully slow if it runs out of RAM, whereas many activities you do on an older Mac with a slower CPU can feel beautifully responsive if they have plenty of RAM to work with.

macOS itself occupies a portion of your RAM, as do apps when they’re running and the documents you have open. Some apps need more RAM than others, but in general the more windows, apps, and documents you have open at once, the greater your memory usage.

However, it’s an oversimplification to say that RAM is either “used” or “free.” macOS uses a sophisticated virtual memory system that intelligently and rapidly swaps chunks of data, sometimes called pages, between physical RAM (where it can be accessed instantly) and “swap files” on your startup disk (which is much slower to access but usually has a great deal more space) as needed. (The process of swapping one or more active memory pages to disk is called a pageout.) In addition, macOS automatically compresses data in RAM that hasn’t been used recently, freeing up more space for other apps; it can expand the compressed data almost instantly when needed. macOS doesn’t wait until all your RAM is full to begin compressing it or swapping pages out to disk, but rather uses both techniques in such a way as to maximize both performance and free RAM.

The result of this design is that you can have a number of apps open that cumulatively need more RAM than you have physically installed. macOS makes it appear to apps as though you have essentially unlimited RAM, and, for the most part, sorts out all the details invisibly, in the background, as you work.

However, as the amount of data that absolutely must be kept in real RAM increases, macOS is forced to swap more and more data between your RAM and your startup disk—in other words, to rely more heavily on virtual memory swap files. If you have no available RAM and a process asks for more, macOS must swap some memory pages to disk, and then swap them back as needed.

This swapping is time-consuming; your disk (even a fast SSD) is much slower than physical RAM, so the more swapping your Mac has to do, the slower its performance becomes. Intensive virtual memory use also produces a secondary slowdown effect: if your disk is busy reading and writing virtual memory swap files, it takes longer to read and write other files, such as the music you’re playing in iTunes, the book you’re writing in Word, or your Time Machine backups. And, if you’re running low on disk space and the swap files grow large enough, the result can be that macOS stalls or crashes because it has nowhere to put data (either in RAM or on your startup disk) that it needs to use.

To summarize: the more physical RAM you have, the more apps and documents you can have open before virtual memory begins to slow down your Mac. Having more RAM won’t speed up your Mac much if it hasn’t yet reached the point where virtual memory has to use your disk heavily. Rather, it reduces the likelihood of your Mac reaching that point—and thus makes a profound and dramatic difference in speed.

At what point do you run out of physical RAM, such that adding more will benefit you? That varies. To find out how your RAM is being used at any given time, read Check Your Resource Usage; for details about increasing the RAM in your Mac, read Add RAM. As a rule, my experience has been that having more RAM usually helps, and never hurts.

Disk Speed

Whereas RAM is used for actively running programs and open documents, your disk—whether a mechanical hard drive or a solid-state drive (SSD)—is used for long-term storage of your software, files, and preferences. But macOS itself and your apps frequently read from and write to your disk. The greater the amount of data to be read or written, the longer it takes, and if a process has to wait for the disk to finish doing its thing before it can perform the next task, the effect will be one of slowness at best, and complete inactivity at worst. So, faster disks, because they can get data on and off in less time, should result in a faster Mac.

But this, too, is a tremendous oversimplification. Among the factors that go into perceived disk speed are these:

• Physical and mechanical characteristics: Hard drives have spinning platters, and, all things being equal, a faster rotational speed equals faster real-world performance. But the number of platters, the density with which data is stored on them, the presence and size of an on-disk cache, the speed with which the read/write heads can move between sectors, the design of the controller used, the interface it uses (for example, SATA, USB, or Thunderbolt) to communicate with your Mac’s logic board, and other design features influence the disk’s overall speed. Plus, some models are hybrid drives—hard drives that include a small, integrated SSD that holds frequently accessed data and thus increases the drive’s overall performance. But note that a given drive may be fast in one scenario (say, reading large chunks of contiguous data) yet slower in another (such as writing large numbers of small files all over the disk).

• Competition for attention: Contrary to appearances, a hard drive can do only one thing at a time. It can’t read files from two different locations while writing a file to a third, for example. What it can do is switch back and forth between activities and locations rapidly, but because this involves lots of physical movement of the read/write head, it makes all the individual search, read, and write processes take longer. This is one reason everything on your Mac may slow down when a backup app is running—your disk is busy reading and writing backup files, and when you ask it to do something else, like play a movie, that forces the disk to switch between tasks and physical locations on the different platters.

  Having plenty of free RAM can sometimes compensate for this effect, as macOS may buffer large amounts of data from disk in RAM (for example, when playing a movie) to minimize disk access.

Up to this point I’ve been talking primarily about mechanical hard drives. SSDs are faster because they have no moving parts. An SSD never has to wait for a read/write head to move to the right spot or for the right part of the platter to spin into place. However, it’s still true that an SSD can read or write data from only one location at a time. If you ask an SSD to do several things at once, it can switch between them more rapidly and efficiently than a hard drive, but even so, only one piece of data will be moving between the drive and the other components in your computer at any moment.

Not all SSDs are created equal, though; because of variations in design and manufacturing, one might be barely faster than a hard drive while another is screaming fast.

Putting It All Together

Not only are CPU power, RAM, and disk speed independently important in determining your Mac’s speed, but they all interrelate in a bunch of ways. For example:

• Any process that uses CPU cycles also uses RAM. So reducing CPU usage almost invariably frees up some RAM too.

• Having lots of free RAM can make non–CPU-intensive tasks faster.

• Having a slow disk (see Choose a New Drive) can make disk-intensive tasks feel sluggish even if you have a powerful CPU.

• The more free RAM you have, the less dependent you’ll be on free disk space for virtual memory to use. Meanwhile, a faster disk (or, better yet, an SSD) will speed up virtual memory swapping.

In short, the ideal situation for Mac performance includes the following elements:

• The maximum possible available CPU power

• Enough free RAM to keep virtual memory swapping to a minimum

• A disk that transfers data to and from your Mac quickly

Unlike many PCs, most Macs don’t have upgradable CPUs, so in general the only way to increase CPU power (apart from buying a new Mac) is to reduce the number or intensity of tasks it’s performing, leaving more power for the tasks that are most critical.

If you regularly run low on free memory and can add more RAM, you should. If your Mac already has the maximum amount of RAM it can hold—or if you can’t afford to add more—the next best thing is to reduce the demand for RAM (for example, by having fewer apps and documents open) so that less virtual memory swapping occurs. And you’ll want to keep enough disk space available to accommodate your virtual memory swap files, as I discuss later in Free Up Disk Space.

As for your disk, you may be able to upgrade to a disk that’s inherently faster (such as an SSD), but even so, your disk performance will decline as you place greater simultaneous demands on it. So the trick is to find ways to get your disk to do fewer things at once.

Those principles form the basis for the majority of the steps I describe in the remainder of this book. If you can free up CPU cycles and RAM, and keep your disk from being overtaxed, you’ll go a long way toward speeding up your Mac.

Clearing Caches

As you use various apps, they often store frequently used information in files called caches. For example, when you visit a webpage in Safari, the browser stores the text and images from that page in a cache, so that the next time you go to the same page it can be displayed more quickly (because Safari needn’t download every bit of it again). Another example is Microsoft Word, which can display the fonts in the Font menu in their own typefaces. If Word had to read in all those fonts each time you used it in order to build the Font menu, every launch could take a minute or more, so Word builds a cache that contains all the data it needs to draw the font names.

Caches are good things—usually. But sometimes they cause more problems than they solve. One example is when an app has cached hundreds or thousands of files—so many that reading in the caches takes longer than reading (or recomputing) the source data they contain, thus slowing down the app instead of speeding it up!

A more serious problem involves damaged cache files. Maybe an app failed to write the file correctly in the first place, or the information it put into the cache was bad, or a disk error corrupted the cache after the fact. Whatever the reason, a corrupted cache file can cause an app to crash, run slowly, or exhibit any number of incorrect behaviors.

In other words, clearing caches can sometimes solve problems—including, occasionally, speed problems—but it’s much more effective as a troubleshooting technique than as a regular speed-enhancing step.

In any case, I recommend against blindly deleting all your caches; as I said, they usually help rather than hinder. However, a few caches in particular have notorious reputations, and clearing them can occasionally help make the apps that use them run more smoothly. I go into considerable detail about these exceptional caches in Take Control of Troubleshooting Your Mac, but the short version is that if you’re experiencing unexplained slowdowns in Safari or Microsoft Office, or unusually slow startup or app launching times (especially when coupled with unexpected font behavior), you might consider clearing the following caches.

Freeing Up Disk Space

It’s true: sometimes you can improve your Mac’s speed by deleting files (a task I discuss later, in Free Up Disk Space). For example, if your startup disk is running desperately low on empty space, your Mac may grind to a halt. So, going from almost no empty space to, say, 10 GB can make a dramatic difference. But if you already have hundreds of gigabytes available, deleting files will usually have little to no perceptible effect on speed.

What, then, is the connection between disk space and speed?

One important consideration is virtual memory. As you use your Mac, apps may request more RAM than is physically present. That by itself isn’t a big deal; the macOS virtual memory system was designed to handle these situations gracefully—within reason. But if you continue opening apps and documents that ask for more and more RAM, macOS responds in the only way it can, by creating an increasing number of virtual memory swap files on your disk.

Excessive virtual memory use itself slows down your Mac (as described in RAM). In addition, the constant disk access makes other disk read and write operations take longer, slowing down macOS itself and every app that tries to read or write data on your disk. Even worse, if macOS runs out of RAM and also runs out of disk space to store virtual memory swap files (even if that’s because too much disk space is already filled with swap files), your Mac gets stuck—it has data that has to go somewhere but there’s nowhere to put it. The result is that your Mac slows to a crawl as the virtual memory system struggles to keep just enough data in physical RAM to keep going.

Your Mac can handle low-memory situations much better if it has plenty of disk space to use for swap files. Conversely, it can handle low disk space more easily if your RAM usage is so small—or, conversely, you have so much RAM—that little demand is placed on disk-based swap files. The combination of low free RAM and low disk space can quickly spiral out of control, and that’s what you want to avoid. Still, you don’t need hundreds of gigabytes of empty space to ensure that virtual memory won’t slow down your Mac; see Free Up Disk Space for details on how much space you truly need and how to obtain it.

Virtual memory isn’t the only connection between disk space and performance, however. Even if your Mac has loads of physical RAM and plenty of empty space on its disk, there is a correlation (albeit a small one) between the amount of available space on a hard disk and the speed with which data can be read and written. (This is true only for hard disks, by the way—not for SSDs.) The reason is that the first files to be written to a spinning hard disk are stored, in contiguous chunks, on the fastest part of the disk. As time goes on, data is written to slower parts of the disk, and files become fragmented, making it more time-consuming to read them. So even a disk with lots of empty space could suffer a certain performance penalty as you add files. That brings us to the next topic: defragmentation and optimization.

Defragmenting and Optimizing Your Disk

As you use your Mac—or, more specifically, as you copy and save files—your files gradually become fragmented into smaller segments scattered across your disk. Some people consider this a serious problem and go to great lengths (and expense) to correct it. Before worrying about fragmentation, you should understand how and why it happens—and what the real-world consequences are.

Pretend, for the sake of illustration, that your hard disk consists of exactly ten blocks and that it initially contains five small files (A, B, C, D, and E), each of which takes up exactly one block. Your disk looks tidy and clean, something like this: ABCDE_⁠⁠⁠.

If you add a couple of new files, F and G, and then delete files B and D, your disk looks like this: A_C_EFG_⁠_⁠_. If you then add a file H that’s twice as big as the others, the drive puts it at the end, like so: A_C_EFGHH_. Now let’s say file G grows to two blocks in size. There being too little space between F and H, G must split into two segments: A_C_EFGHHG. Finally, if you add file J and delete file F, your disk looks like this: AJC_E_GHHG.

Now imagine this happening with hundreds of thousands of files of different sizes. Some tiny files might occupy just one block, while some huge ones may occupy millions of blocks. The more you read and write files, the more jumbled the data becomes: individual files are split into numerous noncontiguous chunks, and lots of small, empty spots develop where other files once lived. That’s fragmentation: the normal state of your hard disk!

Ordinarily, you never notice fragmentation, because macOS keeps track of which parts of which files are where and automatically reassembles them in memory as you open them. With modern hard drives, this process goes so fast that it’s normally imperceptible. Furthermore, macOS includes automatic background defragmentation of smaller (less than 20 MB) files, so that although files may not be contiguous with each other, at least most of them are in one piece.

But a problem occurs when you have apps that must read or write massive amounts of information in real time, such as audio or video recording and editing apps. When these large files become fragmented, the drive’s read/write head must physically zip back and forth over the disk to get all the segments, and sometimes the rate at which it does the zipping is too slow to keep up with the amount of data being saved or read. The results can include gaps in the data, stuttering, or slow app performance. Defragmentation can solve this problem by using special software to reassemble all the files on your disk so that each one occupies a single continuous stretch of space.

Even if all the files on your disk have been defragmented, there can still be gaps between those files. That means that the read/write head still has to do a good bit of skipping around when accessing multiple files and, more important, newly written files will have to fit into those gaps, making it more likely that they’ll be fragmented. So defragmentation software usually goes a step further and rearranges all the defragmented files to eliminate these gaps.

In fact, the story gets even more complex, because the position of a file on your disk also makes a difference in how quickly it can be written or read. The outside or “beginning” of a disk platter (of which any given hard drive may have several) moves past the read/write head at a faster rate than the inside, so files stored closer to the outside of the platter can be accessed more quickly than files stored closer to the middle. For that reason, when you start writing data to a blank disk, it generally fills up the platters from the outside in (not counting all the jumping around necessary due to fragmentation). And the defragmentation feature built into macOS places crucial system data in a “hot band” in the fastest part of your disk.

Likewise, third-party defragmentation utilities nearly always include a step called optimization (sometimes it’s optional, sometimes not), which includes rearranging the files without gaps, toward the outside of the disk platters, with the files you access most frequently stored on the fastest part of the disk.

Defragmentation and optimization can indeed improve a hard drive’s performance, but that improvement is usually quite small unless the fragmentation is severe (as evidenced by long delays in opening and saving files). In addition, if you use high-end audio or video apps regularly, occasional (say, monthly) defragmentation is worthwhile. And if your disk is nearly full, it will tend to fragment files more than a disk with lots of empty space, so defragmentation may be a good idea.

But if you choose to defragment and optimize your drive, be aware of these caveats:

• The process is quite slow. If you’re defragmenting a large disk (especially if it’s your startup volume), your Mac could be effectively out of commission for many hours, or even days.

• Defragmentation is somewhat risky, since it involves deleting and rewriting almost every file on your drive. A good backup is always essential before undertaking defragmentation.

• As Apple cautions in About disk optimization with Mac OS X, defragmentation performed by third-party utilities can potentially undo the optimization macOS itself performs.

• You can’t have a permanently defragmented disk. As soon as you make any changes to an existing file, you’ll introduce some fragmentation. That’s not a problem, but just a fact of the way hard disks work.

• Remember, you should never defragment an SSD (solid-state drive), as this won’t speed it up and can actually reduce its lifespan.

To learn how to defragment and optimize your disk in those situations where it truly makes sense to do so, skip ahead to Defragment and Optimize Your Disk.

Repairing Permissions

For many years, conventional wisdom held that using Disk Utility’s Repair Disk Permissions feature could prevent or cure countless problems, including slow performance. That’s mostly a myth—and it’s also irrelevant for anyone running 10.11 El Capitan or later—but there is a kernel of truth in it.

In macOS, each file contains information specifying which users (or parts of the system) can read it, modify it, or execute it. This information is collectively known as permissions. If a file has incorrect permissions, it’s possible that a user, an app, or the system itself won’t be able to work with the file, which can cause apps or macOS itself to misbehave in various ways, such as crashing or failing to launch.

Ordinarily, installers set the correct permissions for the files they install, and the permissions stay that way permanently. However, a poorly written installer can mess up permissions—even for files it did not install—and if you use Unix commands such as chown and chmod, or even the Sharing & Permissions section of the Finder’s File > Get Info window, you can accidentally set files’ permissions incorrectly. These sorts of problems occur infrequently, but they do occur.

In 10.10 Yosemite and earlier, Disk Utility’s Repair Disk Permissions feature looks for certain software installed using Apple’s installer (which saves files called receipts that list the locations and initial permissions of all the files in a given installation package). It then compares the current permissions of the installed files to those in the receipts and, if it finds any differences, changes the files’ permissions back to match the information in the receipts. The command ignores software installed in other ways (using a different installer or drag-and-drop installation, for instance) and has no way of knowing to refrain from “correcting” permission changes you may have made deliberately.

Starting in El Capitan, Apple has addressed the problem of incorrect permissions in two ways: macOS now protects certain critical directories so that their contents (including permissions) can never be changed except by the Apple installer; and every time you run the Apple installer (such as when you update macOS), the installer checks permissions for Apple-installed files and resets any that are incorrect.

If you’re still running Yosemite or earlier, you could potentially have out-of-whack permissions. But even when permissions are wrong, slow performance is unlikely to be a symptom. So repairing permissions makes little sense if your goal is to speed up your Mac.

I do, however, recommend repairing permissions as a troubleshooting step if (especially right after installing new software) an app no longer launches or it produces inexplicable error messages.

To repair your permissions in Yosemite or earlier, follow these steps:

1. Open Disk Utility (in /Applications/Utilities).

2. In the list on the left, select the volume whose permissions you want to reset, as shown in Figure 1, where I’ve selected my main startup volume. (Permissions can be repaired only for volumes containing a bootable copy of macOS.)

3. On the First Aid pane, click Repair Disk Permissions.

In the details area, Disk Utility displays messages about what permissions, if any, it resets.

Eliminating Malware

PCs running Windows—particularly older versions, such as Windows XP—are notorious malware magnets. If someone running an older PC complains of constant disk access and sluggish performance, one of my first thoughts would be that they may have a malware infection. But there’s far less Mac malware in circulation—so little that most people would have to go to some effort to encounter any. Unless you download pirated software or media, open mysterious email attachments from strangers, click “Your Flash Player Is Outdated” banners (a sure sign of danger), or spend a lot of time on sketchy websites (porn, gambling, warez, and so on), you’re unlikely to encounter malware on your Mac, and even if you did stumble across some, it’s even less likely that it would be the sort that would run in the background and slow down your Mac.

Furthermore, macOS itself includes numerous anti-malware features that operate silently, behind the scenes. As long as you keep your Mac up to date with the latest system software and security updates—and don’t go out of your way to circumvent default security measures—most malware you might encounter will be automatically prevented from running.

For some reason, though, developers of anti-malware software for Mac seem to be doing brisk business. I’ve tried lots of these apps, and many of them are worse than useless. Not only do they often find nothing to remove, but they also bog down your disk as they scan every file—over and over again—and some of these apps interrupt you with frequent alerts of various kinds that don’t relate in any way to real problems.

So, my opinion is that if you’re a reasonably savvy Mac user, you need not bother with anti-malware software—unless your employer insists on it or you engage in unsafe computing habits that put you at increased risk of infection. If you do use anti-malware software, at least set its preferences to run only when you manually request it (or overnight, when your Mac isn’t actively in use), rather than constantly in the background.

Indeed, for some people, removing anti-malware software is a way to speed up a sluggish Mac!

Deleting Unused Preference Panes

Many of the guides I’ve read about speeding up your Mac advise you to remove any third-party preference panes you can live without, based on the theory that they have background processes that suck up unreasonable amounts of CPU power, leaving less of it for everything else.

The advice to remove CPU-hogging programs is good, but most preference panes simply don’t meet that description. For example, on the Mac I’m using to write this paragraph, I have eight third-party preference panes installed. Of those that keep background processes running all the time, their collective CPU usage is, at this moment, about 1%. I’m sure some preference panes do have components that chew up more CPU cycles, but if my experience is at all typical, it’s simply nothing to fret over.

But this isn’t something you have to wonder about. You can check it yourself: follow the instructions in Use Activity Monitor to find out exactly how much CPU power your background processes are using, and then take whatever action you feel is appropriate.

Turning Off Unused macOS Features

If you want to increase your car’s gas mileage (or are in danger of running out of gas), it’s a good idea to turn off your air conditioner, since the extra strain it puts on the engine makes your car use more fuel. By analogy, if you’re running low on CPU power and RAM, turning off things that could deplete those resources should make your Mac go faster. For that reason, many people recommend turning off any feature in macOS that you’re not actively using or can live without. Frequently mentioned examples include Bluetooth, Internet Sharing, and the animation when opening windows.

Here’s what I have to say about these recommendations: No. Just no. These sorts of features use a trivial amount of CPU power—in many cases, none at all when they’re not actively in use. You won’t notice the few milliseconds faster that a Finder window opens. It just isn’t worth it.

Removing Extra Fonts

Macs come with quite a few fonts, third-party packages such as Microsoft Office and Adobe Creative Suite add even more, and you can of course install as many of your own fonts as you like. These fonts take up space on your disk, of course, and if you’re running critically low on space, it’s not a terrible idea to get rid of fonts you never use. But under normal circumstances, fonts generally have no impact whatsoever on your Mac’s speed, and removing extra fonts (as some guides suggest) accomplishes nothing.

There are, as usual, some qualifications:

• As I said in Clearing Caches, a damaged font cache could conceivably cause problems, including slow startup and app launching. But that’s easily solved, and completely separate from the fonts themselves.

• When apps first load fonts, they may spend a bit of extra time caching them, so having more fonts could lead to longer app launch times as the cache is being built. But subsequent launches should be much faster.

• A damaged font file could result in crashes and other bad behavior, and duplicate fonts can in some cases increase startup times. I discuss these two issues in Check Your Fonts.

Avoiding FileVault

FileVault, the full-disk encryption capability built into macOS, is a simple yet effective way to protect the data on your Mac in the event that your computer is lost or stolen. FileVault is optional, but if you turn it on (in System Preferences > Security & Privacy) in order to protect your data, will your Mac’s performance suffer? After all, your Mac does have to do more work encrypting files as they’re written to disk and decrypting them as they’re read.

Apple designed FileVault to be as gentle on your CPU as possible, precisely so you wouldn’t notice a performance hit in most cases. Is there some effect? Of course. Is it the main cause—or even a significant cause—of your Mac performing too slowly? Almost certainly not.

I’ve performed before-and-after benchmarking on two of my Macs, and in both cases FileVault reduced overall performance by less than 1%. For disk-intensive tasks, particularly booting and copying large files, having FileVault turned on can result in a modest speed penalty—and that penalty may be more pronounced if you use a mechanical hard drive rather than an SSD. However, in day-to-day tasks, when you’re not deliberately trying to stress your disk, the performance difference is so slight that you’re unlikely to perceive it.

Personally, I find the security offered by FileVault to be well worth a tiny performance penalty, at least on my MacBook Pro (which is far more likely to fall into the wrong hands than my desktop Macs). And in several years of use I haven’t noticed slowdowns that I can reasonably attribute to FileVault. Of course, your mileage may vary, but I think most people will find FileVault’s advantages to be well worth the small speed hit.